Coating is the process of replacing the gas contacting a substrate, usually a solid surface such as a web, by a layer of fluid. Sometimes, multiple layers of a coating are applied on top of each other. After the deposition of a coating, it can remain a fluid such as in the application of lubricating oil to metal in metal coil processing or the application of chemical reactants to activate or chemically transform a substrate surface. Alternatively, the coating can be dried if it contains a volatile fluid to leave behind a solid coat such as a paint, or can be cured or in some other way solidified to a functional coating such as a release coating to which a pressure sensitive adhesive will not stick.
Often to create the proper functioning of a coated substrate, multiple layers of differing compositions must be applied. There are many examples of this. It is common to apply a primer coating under a paint to improve the anchorage. In manufacturing color photographic film, as many as twelve layers of differing compositions must be applied in a distinct layered relationship with close uniformity tolerances. The manufacture of high performance magnetic recording tapes requires the coating of multiple layers of magnetic pigments of differing compositions.
Sequential coating operations can produce a plurality of distinct superposed layers on a substrate. However, this is costly, time consuming, and may require a large investment in the sequential coating and drying stations.
Methods of applying simultaneous, multiple layer coatings are discussed in Cohen, E. D. and Gutoff, E. B., Modern Coating and Drying Technology, Chapter 4, VCH Publishers, New York, 1992. Slot or extrusion, premetered die coaters are disclosed in U.S. Pat. Nos. 2,761,419 and 2,761,791, and many improvements have been developed over the years. With these coaters, the surface of the web to be coated is brought into contact with or in close proximity to the die surface and a plurality of superposed layers is deposited. Each coating composition is metered to the coating die which deposits the layers on the web. In these methods, the maximum speed of operation is limited and the uniformity of the gap between the die and the web limits the quality of the coatings.
Another method of a simultaneous, multiple layer coating is curtain coating. U.S. Pat. No. 3,508,947 teaches the use of this method with respect to coating photographic elements. Curtain coating uses a free falling vertical curtain of fluid which impinges upon the web traversing the coating station. This patent teaches a method of forming the curtain from a plurality of distinct layers to accomplish a multiple layer coating on the web. The gap between the coating die and the web is much greater than other methods and the speeds of application are substantially greater. However, even this technique has limitations.
To create a multiple layered fluid curtain of miscible layer compositions or coating compositions which have a zero or near zero interfacial tension, the flows of the layers must be kept laminar to avoid mixing. If the preferred slide geometry is used, the maximum flow rate is limited by the transition from laminar to turbulent flow on the slide. If the coating speed is fixed, this limits the maximum coating thickness that may be applied. If the coating thickness is fixed, the maximum speed at which the coating may be applied is limited.
Another limitation of curtain coating is that the free-falling curtain is accelerated by the force of gravity which is constant and limited. The kinetic energy gained in this free fall is used to displace the air from the web surface in a manner to prevent the undesirable entrainment of air. The kinetic energy gain in free fall increases with curtain height, but increased curtain height increases the probability of disturbances to the fragile curtain. In practice, it is difficult to obtain good coating quality with heights above 25 centimeters. This limits the range of thickness and the speed of coating. The desire for high curtains to obtain high speed, thin coatings, and short curtains to obtain high quality coatings are at cross purposes and compromises must be made which restrict the utility of this method. Also, curtain coaters can not function in low or zero gravity environments.
Another limitation of curtain coating is that the curtain always falls vertically under the influence of gravity. This limits the coating station geometries and the coating station orientation. Also, if curtain coating is to be added to an existing manufacturing process, the process must be adapted to the restrictive vertically falling orientation of the curtain rather than orienting the coating die and apparatus to the existing web path of the existing process.
The axisymmetric coater of U.S. Pat. No. 4,348,432 teaches how to form a multiple layer radially expanding sheet from opposed impinging cylindrical multiple layer jets, and how to translate a web past the device to effect a simultaneous, multiple layer coating. However, in addition to the other limitations, this method is severely limited by the maximum web width limitation imposed by the flow dynamics. Widths larger than 1 meter are prohibited and widths larger than 0.75 meters are impractical.
Single layer orifice existing kinetic of fluid issuing from slots are known in the paper industry either to apply an excess of coating liquid to a web surface before metering with a blade coater or to apply an excess of coating liquid to the knurl roll of a gravure coater.
However, no use of multilayer jet coating is known. There is a need for a system that can apply thin multilayer coatings simultaneously, at higher speeds without the orientation, geometric, and gravitational constraints of known coating methods. There is a need for an improved system which can simultaneously coat multiple layers on a substrate with each layer being precisely metered and distributed across the width of the substrate while maintaining the substrate in a controlled juxtaposed face-to-face relationship.